1. Field of the Invention
This invention relates to a color photographic optical system wherein the occurrence of color shading can be avoided.
2. Prior Art
In the prior art, a color photographic optical system is known which uses a three prism type separation optical system as shown in FIG. 11. In FIG. 11, 1 is a color separation optical system which has three prisms 2, 3 and 4 joined to each other. There is a very thin aero layer between the prisms 3 and 4. An interference film 5 is provided prisms 2 and 3, and an interference film 6 is provided between prisms 3 and 4. These interference films 5 and 6 act as a dichroic filter. When a light beam incident on the color separation optical system 1, it is separated into three beams, red (R), green (G) and blue (B) due to the transmitting and reflecting characteristics of the interference films 5 and 6. The light beams are guided respectively to the imaging surfaces of CCD 7, 8 and 9 which act as photographic devices, and images are formed on the surfaces. CCD 7, 8 and 9 output R, G and B signals depending on the amount of light received.
In general, however, if the angle of the light beam incident on the color separation optical system varies, the reflectance and transmittance of the interference films 5 and 6 also vary. Further, the larger the angle of the light beam incident on the color separation optical system with respect to the interference films, the more the transmittance characteristics of the interference films are shifted towards the short wave end of the color spectrum.
First, we will assume that the image output of the light beam formed in the center of CCD 7, 8 and 9 is properly balanced. A light beam incident at an oblique angle (e.g. .theta.) with respect to the photographic optic axis 10 forms an image at the peripheries of CCD 7, 8 and 9.
The transmittance and reflectance of the interference films for light which forms an image in this peripheral area is different from the transmittance and reflectance of the interference films for light which forms an image in the central area of CCD 7, 8 and 9, and therefore the output balance from CCD 7, 8 and 9 for light forming an image in the peripheral area is upset. This causes the occurrence of color shading where color unevenness appears at the edges of a television screen.
This is due to the fact that, considering only the chief ray of the beam for example, the angle of inclination of the interference films 5 and 6 is different for the central area and peripheral area of CCD 7, 8 and 9.
In color TV cameras having a three prism type color separation optical system 1, a possible way of correcting for color shading is to incorporate a correction circuit in the signal processing unit. Color shading can also be corrected by the following method.
First, the position of the exit pupil is fixed by setting conditions such as the magnification of the optical system and aperture diaphragm, and the incidence angle of the chief ray of the beam incident on the color separation optical system 1 with respect to the photographic optic axis 10 is computed. The computed values are then corrected for color shading by applying a correction to each picture element of the CCD 7, 8, 9 for R, G, B.
However, in the former type of color TV camera, the signal processing unit is bulky and the color balance has to be reset every time the photographic conditions are modified. In the latter type of color TV camera, on the other hand, an operational circuit is required which makes the camera costly.